Communications buses are often required to provide low bus acquisition latency for high priority communications and to support large block data transfers. These requirements usually conflict and reduce the efficiency of the transfer of data. One prior art bus system allows for the transfer of data over the bus without permitting any interruptions from a device having a higher priority. This system has an extremely high rate of data transfer. However, this system while less expensive and less complex, is not capable of meeting the bus acquisition latency requirement of real time systems since a low priority bus master cannot relinquish control of the bus to a device having a higher priority in a timely manner. A second solution to the above identified conflict is to (1) break large data blocks into smaller blocks and transmit the blocks as a plurality of short messages or (2) packetize the data into sub-blocks and provide a method of interrupting the transfer thereof at one of the boundaries of a packet. Since the block data transfer can only be interrupted at the end of a message or data packet, the bus acquisition latency requirement places an upper limit on the message or data packet size. Therefore, this solution introduces unnecessary overhead expense in that it requires the transmission of multiple message headers or performing what may be unnecessary data packetization. Moreover, this overhead expense exists irrespective of a higher priority request for the bus during a block data transfer. This is an important consideration since, although the allowable bus acquisition latency is often small, which requires a large number of data packets to transmit a block of data, the number of high priority messages that require the block data transfers to be interrupted is also generally small resulting in a high packetization overhead.
Consequently, there is a need for a bus system which provides low bus acquisition latency for high priority communications and supports large block data transfers without having a high overhead.